Fix a final (?) task field issue which slipped through the cracks

Summary:
Derped this one up; while my testing was successful in preventing runaway
attaching I missed the bit where it doesn't actually work.

This resolves the "Unknown Object" link seen on T661.

Test Plan:

• Created two new revisions, each attached to a local task.
• Verified that they attached additively, Maniphest and Differential were

linked to the right places, and nothign else bad happened.

Reviewers: btrahan, fratrik

Reviewed By: fratrik

CC: aran, fratrik, btrahan

Differential Revision: 1181